This invention relates to a thin-film magnetoresistance sensor and, more particularly, to such a sensor having reduced side reading and improved magnetic stability.
A magnetoresistance (MR) sensor is used in a read/write head to read magnetic fields on a recording medium of a magnetic storage device. An example is the read/write head of a computer hard disk or a magnetic recording tape drive. The read/write head of the computer hard disk is positioned closely adjacent to the recording medium, separated from the recording medium by an air bearing that does not allow them to touch. A data bit is written onto an area, usually a track, of the recording medium using the writing portion of the read/write head by locally changing its magnetic state. That magnetic state is later sensed by the MR sensor to read the data bit.
Two known types of MR sensors are a giant magnetoresistance (GMR) sensor and a tunnel magnetoresistance (TMR) sensor. In general, the sensors are multilayered thin-film devices that sense the magnetic state of the adjacent region of the recording medium. The general technical basis, construction, and operation of the GMR sensor are described, for example, in U.S. Pat. No. 5,436,778. The general technical basis, construction, and operation of the TMR sensor are described, for example, in U.S. Pat. No. 5,729,410. The disclosures of both patents are incorporated by reference in their entireties. These patents also describe the read/write heads and the magnetic storage systems.
There is an ongoing trend to increase the amount of information stored on the magnetic storage device. The amount of information stored may be increased by decreasing the size and spacing of the data tracks of the recording medium in which the information is stored on the magnetic storage device. The size of the MR sensor must be correspondingly decreased and its spatial resolution increased.
Problems arise as the MR sensor is made smaller. One problem is xe2x80x9cside readingxe2x80x9d, where the MR sensor detects the magnetic state of the track of the recording medium directly below the MR sensor, as intended, but also detects some signal from the laterally adjacent tracks. The side reading signal may result in an erroneous reading of the track that is intended to be read. Another problem is that the magnetic stability of the MR sensor may be reduced as the sensor is made smaller.
There is a need for modifications to the design of the MR sensor to reduce the incidence of side reading and magnetic instability. The present invention fulfills this need, and further provides related advantages.
The present invention provides a magnetoresistance (MR) sensor and a method for its fabrication. A side portion of the free layer of the MR sensor is pinned so that it cannot contribute to side reading and to magnetic destabilization. In one approach, the MR sensor has a longitudinal hard biasing structure that exchange couples to the free layer of the MR sensor to pin the free layer. As a result, the side portions of the free layer adjacent to the lateral sides of the MR sensor do not contribute to side reading or magnetic instability of the MR sensor. The present approach may be readily implemented by available fabrication techniques previously used for other purposes. It is applicable to both giant magnetoresistance (GMR) sensors and tunnel magnetoresistance (TMR) sensors.
In accordance with the invention, a magnetoresistance (MR) sensor, such as a giant magnetoresistance (GMR) sensor or a tunnel magnetoresistance (TMR) sensor, comprises a substrate and a sensor structure contacting and deposited upon the substrate, and having a lateral side. The sensor structure comprises a layered transverse biasing structure, and a free layer contacting the layered transverse biasing structure. The free layer has a central portion that is magnetically free to respond to an external magnetic field, and a side portion extending toward the central portion from the lateral side and which is magnetically pinned so that it is not free to respond to the external magnetic field. Optionally, a cap layer overlies at least a portion of the free layer.
A longitudinal hard biasing structure contacts at least the lateral side of the sensor structure. The side portion of the free layer is preferably pinned by exchange coupling from the longitudinal hard biasing structure. The longitudinal hard biasing structure desirably comprises a seed layer contacting the substrate, the lateral side of the sensor structure, and the side portion of the free layer. The seed layer has a seed layer crystal structure, which is preferably body centered cubic (BCC), and is a magnetic material. The longitudinal hard biasing structure further includes a magnetic hard bias layer contacting the seed layer. The hard bias layer has a hard bias layer crystal structure which is preferably hexagonal close packed (HCP) with the Z-axis in the plane of the film. Most preferably, the seed layer is a BCC CoFeCr alloy, and the hard bias layer is an HCP CoPtCr alloy. There is additionally an external interconnection to the sensor structure.
In one embodiment, the magnetoresistance sensor comprises a substrate, and a sensor structure deposited upon the substrate and having a first lateral side and a second lateral side. The sensor structure comprises a layered transverse biasing structure, a free layer deposited upon the layered transverse biasing structure, and optionally a cap layer deposited upon a central portion of the free layer but not upon a side portion of the free layer adjacent to each lateral side thereof. A first longitudinal hard biasing structure is laterally adjacent to the first lateral side of the sensor structure, and a second longitudinal hard biasing structure is laterally adjacent to the second lateral side of the sensor structure. Each longitudinal hard biasing structure comprises a seed layer deposited upon the substrate, the respective lateral side of the sensor structure, and the respective side portion of the free layer. The seed layer has a seed layer crystal structure, which is preferably body centered cubic, and is a magnetic material. The longitudinal hard biasing structure also includes a magnetic hard bias layer deposited upon the seed layer. The hard bias layer has a hard bias layer crystal structure which is preferably hexagonal close packed. Features discussed for other embodiments may be used in conjunction with this embodiment, where appropriate.
A method of fabricating a magnetoresistance sensor comprises the steps of providing a substrate, and depositing a sensor structure upon the substrate. The sensor structure has a lateral side and comprises a layered transverse biasing structure, and a free layer in contact with the layered transverse biasing structure. The free layer has a central portion and a side portion adjacent to the lateral side. A longitudinal hard biasing structure is deposited contacting the sensor structure by the step of depositing a magnetic hard bias layer overlying the lateral side of the sensor structure and the side portion of the free layer but not the central portion of the free layer. Preferably, a magnetic seed layer is first deposited upon the substrate, the lateral side of the sensor structure, and the side portion of the free layer, and then depositing the magnetic hard bias layer upon the seed layer. The seed layer has a seed layer crystal structure that is preferably body centered cubic, and the longitudinal hard bias layer has a hard bias layer crystal structure which is preferably hexagonal close packed.
In a conventional MR sensor, the greatest contribution to side reading and magnetic instability arises from the magnetic response of the portion of the free layer that is adjacent to the lateral sides of the MR sensor. The present approach inhibits the side portion from responding to magnetic fields other than those positioned directly below the central portion of the free layer. It also inhibits the formation of domain walls near the sides of the free layer, which when present contribute to magnetic instability.
The side portion of the free layer desirably has a width exceeding about 0.05 micrometers. If the width is less than this value, there may be insufficient exchange coupling to the face of the free layer to pin the side portion. This width is determined in the fabrication processing.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.